Personal computer systems are well known in the art. Personal computer systems in general, and IBM Personal Computers in particular, have attained widespread use for providing computer power to many segments of today's modern society. Personal computers can typically be defined as a desktop, floor standing, or portable microcomputer that is comprised of a system unit having a single central processing unit (CPU) and associated volatile and non-volatile memory, including all RAM and BIOS ROM, a system monitor, a keyboard, one or more flexible diskette drives, a fixed disk storage drive (also known as a "hard drive"), a so-called "mouse" pointing device, and an optional printer. One of the distinguishing characteristics of these systems is the use of a motherboard or system planar to electrically connect these components together. These systems are designed primarily to give independent computing power to a single user and are inexpensively priced for purchase by individuals or small businesses. Examples of such personal computer systems are IBM's PERSONAL COMPUTER AT (IBM PC/AT), IBM's PERSONAL SYSTEM/1 (IBM PS/1), and IBM's PERSONAL SYSTEM/2 (IBM PS/2).
Personal computer systems are typically used to run software to perform such diverse activities as word processing, manipulation of data via spread-sheets, collection and relation of data in databases, displays of graphics, design of electrical or mechanical systems using system-design software, etc.
At the most fundamental level, a computer's digital logic is comprised of transistors. Individually, these transistors act as simple switches, i.e. allowing electrical current to flow when a transistor is in a first state and not allowing any current to flow when the transistor is in a second state. Collectively, these transistors, or switches, may be connected in certain combinations to construct logical functions such as AND, OR, NOR, NAND, etc. They may also be combined on a more complex level to construct memory registers, flip-flops, programmable logic arrays (PLA's) and microprocessors. All these circuits are sometimes referred to as employing "transistor logic."
Of the many areas in which transistor logic is important, microprocessor development remains in the forefront. This is because a typical microprocessor includes millions of transistors in its digital circuitry. High end microprocessors, such as the Intel PENTUIM.RTM. or the POWER PC, are illustrative.
On a macroscopic level, these transistors perform all of a microprocessor's functions through digital logic circuits. On a microscopic level though, these transistors implement simple logical functions (OR, AND, NOR, etc) through well-known circuit configurations.
Since circuit speed (i.e. signal propagation through a circuit) is a very important consideration in circuit design, CMOS (Complementary Metal Oxide Semiconductor) technology has been traditionally employed to implement these logic functions because of its excellent switching characteristics and high reliability.
However, when a logic function, such as a NOR, requires a large number of inputs, implementation of the logic function through traditional CMOS technology has proven disadvantageous because the large number of inputs required tends to decrease the circuit's speed. To combat this disadvantage, designers have employed ratioed logic or pseudo-NMOS logic (N-channel Metal Oxide Semiconductor).
Illustrative of a ratioed logic circuit is a pre-discharged ratioed logic NOR gate. In a pre-discharged ratioed logic NOR gate, a single P-channel Field Effect Transistor (hereinafter PFET) is connected to the output of the gate along with an N-channel Field Effect Transistor (hereinafter NFET) for every input tied in parallel to ground. A ratioed logic circuit implies that PFET's and NFET's are contesting each other on a particular node when any one or more of the NFET's are "on." The particular node then produces a down level, called a "weak zero" because it is not at exactly ground voltage. This "weak zero," however, still can be interpreted by any logic circuit that follows, e.g. an inverter gate, as a ground level signal. The term "pre-discharged" generally refers to a method in which the output of a logic circuit is charged to ground potential (i.e. 0 volts) during a clock or RESET cycle. Conversely, other digital logic circuits may employ a procedure called "pre-charging" in which the output of logic circuit is charged to a certain voltage potential other than ground during a clock or RESET cycle. A detailed description of a typical pre-discharged ratioed logic circuit will be described hereinafter. To those skilled in the art, it will be apparent that similar circuits can be created using pseudo-PMOS logic (P-channel Metal Oxide Semiconductor). These pseudo-NMOS and pseudo-PMOS logic circuits are all collectively referred to as ratioed logic circuits because they all contain a particular output node which is in effect, ratioed in voltage between a PFET and a competing NFET.
As will become evident, ratioed logic circuits have several advantages as well as disadvantages. The advantages include their fast evaluation speed and compact physical layout characteristics, two very important characteristics for high density, high speed semiconductor products such as microprocessors, memory's, and PLA's. The major disadvantage, however, is that ratioed logic circuits consume large amounts of DC power due to the node contesting of PFET's and NFET's.
The consumption of large amounts of DC power is not only undesirable because it wastes power, but it also contributes to electromigration and hot-electron effects. The term electromigration (EM) refers to the transport of mass in metals when stressed to high current densities. EM occurs during the passage of direct current (DC) through thin metal conductors in integrated semiconductor circuits, and results in accumulation of metal in some regions and voids in other regions. EM in extreme cases may result in failed circuit performance if either (i) accumulations become so severe as to bridge adjacent conductors, thereby causing short circuits, or (ii) voids become so severe as to cause open circuits. Such occasions are referred to as catastrophic faults.
The term hot-electron (HE) effect refers to the phenomenon of electrons which originate from FET surface channel currents, from impact ionization currents at the FET drain junction, or from substrate leakage currents. Electrons drifting from the gate may gain sufficient energy to enter into the gate, or they may collide with the silicon atoms and generate electron-hole pairs. The hole adds to substrate current, and the secondary electron may be injected into the gate of a subsequent FET (see e.g., M. Annaratone, H.B. Digital CMOS Circuit Design, Kluwer Academic Publishers, Norwell Mass., p. 39, (1986)). As these secondary electrons accumulate in the gate, the FET threshold voltage shifts and the internal resistance of the device changes. These changes cause the FET to degrade and ultimately fail over time.